302 Darwin, and after Darwin. 



level of mediocrity is itself a sinking level in successive 

 generations, with the result that there is no reason why the 

 reducing power of panmixia should ever become exhausted, 

 save that the more reduction it effects the greater is the 

 force of heredity which remains to be overcome, as 

 previously explained. Thus the only question between 

 Professor Lloyd Morgan and myself is Does the level of 

 mediocrity fall in successive generations under the cessation 

 of selection, or does it remain permanently where it used to 

 be under the presence of selection ? Does the " birth-mean " 

 remain constant throughout any number of generations, 

 notwithstanding that the sustaining influence of selection 

 has been withdrawn ; or does it progressively sink as a con 

 sequence of such withdrawal ? 



In order to answer this question we had better begin by 

 considering now the case of organization of structure, as 

 distinguished from mere size of structure. Take any case 

 where a complex organ such as a compound eye has been 

 slowly elaborated by natural selection, and is it not self- 

 evident that, when natural selection is withdrawn, the com- 

 plex structure will deteriorate ? In other words, the level of 

 mediocrity, say in the hundred thousandth generation after 

 the sustaining influence of natural selection has been with- 

 drawn, will not be so high as it was in the first generations. 

 For, by hypothesis, there is now no longer any elimination 

 of unfavourable variations, which may therefore perpetuate 

 themselves as regards any of the parts of this highly complex 

 mechanism ; so that it is only a matter of time when the 

 mechanism must become disintegrated. I can scarcely 

 suppose that any one who considers the subject will question 

 this statement, and therefore I will not say anything that 

 might be said in the way of substantiating it. But, if the 

 statement be assented to, it follows that there is no need to 

 look for any cause of deterioration, further than the with- 

 drawal of selection or cessation of the principle which (as 



